The present invention relates to a photographic base material which contains a coating on at least one side, which coating includes a binder component that is produced predominantly from unsaturated compounds and that has been hardened by means of energy supplying radiation. The invention relates especially to a base material for photographic layers that can be used for dry developing processes as well as for wet developing processes.
A multitude of base materials for photographic coatings are known. The most widely used bases include film materials and paper. As a rule, papers are additionally coated. Waterproof papers coated with synthetic resin layers have been widely used as bases for photographic coatings. Films of synthetic resin coated with special layers and laminates also are known. Most widely used are papers that are coated on both sides with polyolefin resin and film materials coated with pigmented layers, such being described, for example, in DE-AS No. 14 47 815, U.S. Pat. Nos. 3,833,380, 3,630,742, 3,928,037.
Especially advanced paper bases which consist of base paper with at least one coating produced in situ by means of electron radiation hardening are described in DE-OS No. 30 22 451 and DE-OS No. 30 22 709. The advantages of a coating hardened with electron radiation are its resistance to marring, its great ability to take on pigments, and its enhanced surface smoothness when compared with polyolefin coatings.
Coatings of synthetic resin are resistant not only to water but also to acid and alkaline photographic process solutions to thereby prevent the penetration of these solutions into paper supports that may be lying thereunder. Consequently, for paper or a mat base, the time-consuming washing process is significantly shortened. However, the widely used synthetic resin coatings consisting of polyolefins are generally less stable with respect to the effect of heat thereon because the coating material is thermoplastic.
All resin coatings, whether they are on paper or on film, can contain pigments, coloring substances, optical brighteners, picture stabilizers, antioxidants or other additives to the extent that they are desirable or necessary in view of the desired characteristics of the photographic picture to be present on them. Of these types of possible additives, pigments and coloring substances have the greatest importance for the visual impression of a photographic picture present on the coating. For color pictures, for example, they determine the color character and are decisive in establishing the image quality of the photographic pictures.
After a suitable pretreatment of the resin surface, the photographic coatings are applied on the surface of the resin layer either directly or after a previous application of an adhesion promoting or producing intermediate layer. These photographic layers are preferably layers of the type that have been known in connection with the concepts of silver salt photography, whether to produce either black-and-white pictures or color pictures. Silver salt photography concepts comprise not only photographic coatings, which contain inorganic silver salts and are developed predominantly wet with so-called developing solutions or pastes, but also photographic coatings which contain organic silver compounds and are developed predominantly dry, for instance, by the effect of heat.
According to DE-OS No. 30 22 451 or DE-OS No. 30 22 709, resin coatings hardened by electron radiation are produced by the application of a flowable mixture on the surface of a paper or on another base support by distributing the mixture thereon evenly and subsequently under protective gas or another covering by solidifying it by means of energy-rich electron rays. The mixtures contain as determining components at least one substance with ethylenically unsaturated double bonds which have the ability to undergo a polymerizing reaction with one another if this is initiated by radiation.
Suitable substances with ethylenically unsaturated double bonds are vinyl compounds or vinylidene compounds. Preferably, the mixture should contain at least one substance which contains two or more double bonds. Additionally, however, non-reacting substances without double bonds also can be contained in the mixture to a limited extent.
Especially suitable components capable of reaction are those substances which contain acrylate groups or methacrylate groups. Also suitable for use as reactive mixture compounds are esters of maleic acid, fumaric acid, mesaconic acid, citraconic acid or itaconic acid, other derivatives of these acids, allyl compounds, as well as linear and cyclic dienes or trienes, respectively. Preferred esters of these acids are those that are derived from multivalent alcohols. Examples are hexanediol diacrylate, trimethylol propane triacrylate, polyester acrylate, polyurethane acrylate, polyether acrylate, polyepoxy acrylate, alkyd resin acrylate, or the methacrylates corresponding to these acrylates. The flow characteristics of hardenable mixtures are adjusted by mixing components having a high molecular weight with components having a lower molecular weight. The hardness and flexibility of the hardened layers is determined by the proportion of double bonds to the molecule size present in the starting material and can be varied over a wide range by mixing of different substances with one another.
To be sure, layers hardened with electron radiation have many advantages when compared with polyolefin layers, but heretofore their usefulness has been seriously hampered by certain disadvantages. In spite of the great breadth of variation in the composition of radiation-hardened coatings, up until now coatings could not be produced which exhibit equally good behavior in all photographic process solutions. Rather, it was found that the layers hardened with electron rays behave differently with respect to different process solutions. Commercial color developer preparations have the disadvantage of a following oxygen effect that leads to a yellowish discoloration of the surface of the radiation-hardened layer. Although this coloration typically is slight, it is clearly visible and cannot be prevented, either by the shortstop bath or fixing bath or by thorough washing. Discoloration occurs especially in connection with the use of such photographic developing solutions which contain an aromatic amine derivative, such as a derivative of phenylene diamine, toluidine, and others. Consequently, the use of papers or films with radiation-hardened coatings has been restricted up until now to such processes in which other chemicals are used for developing the picture, for instance hydrochinone, or in which the picture is developed dry.
The described discoloration of the surface was observed first on coatings which had been produced from acrylic acid esters. Use of methacrylic acid esters or allyl compounds in place of the more widely used acrylic acid esters also was found to be unsatisfactory. All layers were clearly stained after treatment with certain commercial developing solutions which contained aromatic amine derivatives. The measured change of density was predominantly between 0.03 and 0.1, but greater changes also occurred.